1. Field of the Invention
The invention relates to composites including outer metal layers and an intermediate viscoelastic layer for sound and vibration damping characteristics and, in particular, to such composites which are resistance-weldable.
2. Description of the Prior Art
Composites with a thin viscoelastic layer between two metal layers have been used to attenuate noise and vibration and have been used in a number of environments where noise is a problem, especially to diminish the propagation of structural noise and the transmission of air-borne noise. Such environments include automobiles or other vehicles, machinery, business equipment, appliances, power equipment and the like. It is known to provide weldable composites wherein the viscoelastic layer has distributed therein particles of an electrically conductive material to facilitate electrical conduction through the composite, thereby to permit resistance welding of the composite. Such a composite is disclosed, for example, in U.S. Pat. No. 4,978,582.
In many of these applications, it is important that the metal layers of the composite be galvanized to protect them from rust, corrosion and the like. Currently, sound-damped, galvanized composites comprise two sheets of pre-galvanized metal separated by a viscoelastic layer. If the metal layers are pre-galvanized by hot dip galvanizing, then, since each metal layer has the galvanizing metal on both sides, the composite has four layers of zinc, zinc alloys or other galvanizing metal, which makes the composite difficult to weld through. It is possible to electro-galvanize only one side of each metal layer and then laminate the non-galvanized sides together, but this is much more expensive and it is difficult to deal with the thin metal layers used.
Heretofore, a weldable sandwich of two metal layers separated by a viscoelastic layer could not be galvanized after formation of the sandwich. It could not be hot dip galvanized, since the viscoelastic layer would not survive the galvanizing temperature. It could not be electro-galvanized since, in standard electro-galvanizing processes, one surface of the web to be galvanized is passed in electrical contact with a charged conductive roller. In order to plate both outer surfaces of the sandwich, it is necessary for the electric current to pass through the viscoelastic layer to the other metal layer. However, the conductivity of the viscoelastic layer when the material is at rest has typically been insufficient to permit effective electroplating of the outer surfaces of both of the metal layers of the sandwich.